Most of the elements in the periodic table are metals. Metals have "free" electrons—electrons that flow in an electric field—so they conduct electricity well, they reflect light, and, viewed with the light behind them, they are opaque. The metals used in product design are, almost without exception, alloys. Steels (iron with carbon and a host of other alloying elements to make them harder, tougher, or more corrosion resistant) account for more than 90% of all the metals consumed in the world; aluminum comes next, followed by copper, nickel, zinc, titanium, magnesium, and tungsten, in the order that was illustrated in Figure 2.1.
Compared to all other classes of material, metals are stiff, strong, and tough, but they are heavy. They have relatively high melting points, allowing some metal alloys to be used at temperatures as high as 2200°C. Only one metal—gold—is chemically stable as a metal; all the others will, given the chance, react with oxygen, sulfur, phosphorous, or carbon to form compounds that are more stable than the metal itself, making them vulnerable to corrosion. There are numerous ways of preventing or slowing this corrosion to an acceptable level, but they require maintenance. Metals are ductile, allowing them to be shaped by rolling, forging, drawing, and extrusion; they are easy to machine with precision; and they can be joined in many different ways. This allows a flexibility of design with metals that is only now being challenged by polymers.
Primary production of metals is energy intensive. Many, among them aluminum, magnesium, and titanium, require at least twice as much energy per unit weight (or five times more per unit volume) as commodity polymers. But most metals can be recycled efficiently, and the energy required to do so is much less than that required for primary production. Some are toxic, particularly the heavy metals—lead, cadmium, mercury. Some, however, are so inert that they can be implanted in the body: stainless steels, cobalt alloys, and certain alloys of titanium are examples. Here are data sheets for the 12 metals and alloys in the order in which they are listed Table 12.1.